Sky & Telescope - October 2022 - 37

VATICAN OBSERVATORY
Webb also cites other notable scientists, including German
astronomer Rudolf Engelmann, England's John Herschel,
and the famous Prussian lunar observers Wilhelm Beer and
Johann Heinrich von Mädler. He even quotes Secchi, who
described seeing the shapes of these moons as " irregular and
elliptical. " Indeed, many observers of the era reported various
surface features on these satellites.
What are the chances they were seeing real features on
Jupiter's moons? Not good. Perceiving the satellites as disks
(as opposed to mere points of light) is one thing; seeing features
on those disks is quite another. As most readers know,
all telescopes have an inherent limit in their ability to resolve
fine detail that depends on the diameter of the objective lens
or mirror. Secchi's telescope had an aperture of 9.6 inches
(24 cm), which gives it a theoretical resolution of about 0.5
arcsecond. Ganymede, the largest Jovian moon, never appears
larger than 1.8 arcseconds across (1
/1000 the diameter of the
full Moon). To put it simply, seeing detail on Jupiter's moons
requires a large telescope used under extremely favorable
conditions (S&T: Jan. 2014, p. 54).
Despite the limitations of their instruments, 19thcentury
observers reported diameters for the moons within
10% of their actual values (accurate to about 1
/10
arcsecond).
Secchi's numbers, however, reflect a precision of 1
/1000
arcsecond, and one of his Jovian moon drawings shows what
appear to be polar caps like those seen on Mars - certainly
not real features.
The preferred theory of the time to explain the moons'
varying brightnesses invoked pancake-shaped objects that
tumbled - one moment we might be seeing them edge-on
and dim, and then later face-on and bright. And when they
were in between edge-on and face-on, we'd see the moons as
ellipses. Regardless of how strange this might sound today,
at the time some astronomers truly thought they were seeing
The Color of Ice
Why should low density and high
albedo indicate the presence of water
ice, instead of some other exotic
material?
Firstly, water ice should be really
common in the universe. This
comes from knowing what the stars
themselves are made of. Nineteenthcentury
pioneers in spectroscopy
figured out how to split a star's light
into its component colors, thus determining
what elements are present.
Angelo Secchi identified the spectral
signature of hydrogen gas, and by
the early 20th century, astronomers
understood that stars are mostly
composed of hydrogen, with helium
coming in a distant second.
The most common elements after
hydrogen and helium are oxygen,
carbon, and nitrogen. Thus, the most
common compounds in planets turn
out to be water, methane, and ammonia
- the chemicals that result
from hydrogen reacting with oxygen,
carbon, and nitrogen, respectively.
In the outer solar system these
compounds all freeze into ices. The
easiest of these to freeze is water
- we have snow and ice on Earth,
at temperatures where methane
and ammonia remain gasses. So it's
reasonable to identify the low-density,
high-brightness material in the
moons of Jupiter as water ice.
In fact, water ice has a very distinctive
spectral signature in infrared
light. That was finally detected in
the light from Jupiter's moons and
Saturn's rings in the early 1970s, by
competing teams of astronomers at
the Massachusetts Institute of Technology
and the University of Arizona.
sk yand tele scope .o r g * OCTOBER 2022 37
significant variations in the moons' brightnesses. And that
would have made it impossible to determine their albedos,
and therefore whether they were icy or not.
And what about density? Authors of astronomy books in
the 19th century would often list each Jovian moon's mass
and radius, but not bother to divide the mass by the volume
to calculate their densities. The notable exception was
English amateur astronomer George Frederick Chambers.
His 1861 A Handbook of Descriptive and Practical Astronomy
contains a wonderful table of sizes, masses, and densities
for Jupiter's moons. However, his density figures look very
odd - they're all only about 1
/10 the density of water. Little in
nature has that density. Gasses are far less dense than that.
Everything else, even objects that float in water, are closer to
water's density. Yet Chambers simply offers this information
without comment.
What's surprising is that the numbers Chambers lists for
size and mass are pretty good - close enough to have allowed
him to compute moon densities that wouldn't be too far
COLOR VISION The spectra of various stars as shown by the 19thcentury
Italian astronomer and pioneering spectroscopist Angelo Secchi.
Presented here from top to bottom are the spectra of the Sun, Sirius,
Betelgeuse, and Alpha Herculis (Rasalgethi). This work was the first systematic
effort to classify stars by their spectra - Secchi observed about
5,000 stars - which led ultimately to the Hertzsprung-Russell diagram
and our understanding of stellar evolution.
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